Haemophilus pleuropneumonia (Matthews and Pattison, 1961; Shope, 1964) should now be accepted as the valid name of the pathogen of hemophilus pleuropneumoniae of swine, according to studies and the description by Kilian and co-workers (1978), while the name of Haemophilus parahaemolyticus should be considered a synonym. On top of Haemophilus pleuropneumoniae, Haemophilus parasuis and Haemophilus spec. "minor group" are also important to pneumonia, serositis, and arthritis (Glässer's disease) in swine, whereas Haemophilus suis is hardly any longer isolated. Evidence as to dependence on growth factors and to haemolysin activity is not sufficient in all cases for high-accuracy differentiation between those four haemophilus species. Such tests have to be supplemented with evidence as to separation of urea, xylose, lactose, and mannite or, in extraordinary cases, even with evidence to fermentation of melibiose and D-ribose, Serological typing is another tool for further differentiation. Pathologico-anatomic changes in terms of haemorrhagic-necrotising pleuropneumonia are caused by what is called Pasteurella haemolytica-like organism (Bertschinger and co-workers, 1978; Pohlenz and co-workers, 1978) rather than by haemophilic bacteria only. The authors, using the above pathogens, succeeded in causing in specific-pathogen-free pigs (SPF) manifest pleuropneumonia which could not be distinguished from haemophilus pleuropneumonia. Pathologically and bacteriologically, those germs are of great relevance to differential diagnosis, in the context of haemophilus pleuropneumonia.